(1-1/2²)(1-1/3²)(1-1/4²)(1-1/5²)……(1-1/2004²)(1-1/2005²） =(1+1/2)(1-1/2)(1+1/3)(1-1/3)……(1+1/2004)(1-1/2004)(1+1/2005)(1-1/2005)=(3/2)(1/2)(4/3)(2/3)(5/4)(3/4)(6/5)(...